The continual drive towards agile, facile methods for producing near-net shape advanced ceramic components has led to a revolutionary class of forming techniques known as solid freeform fabrication (SFF). Such techniques utilize computer-controlled robotics to build three-dimensional components in a layer-by-layer fashion. To date, several SFF techniques, including stereolithography (SLA), fused deposition of ceramics (FDC), laminated object manufacturing (LOM), computer-aided manufacturing of laminated engineering materials (CAM-LEM), 3D printing (3DP(trademark)), and robocasting, have been developed. However, many of these approaches rely on feedstocks that contain 40-70 vol % organic species, which give rise to debinding problems and limit component sizes to xcx9c2 cm thickness or less.
Robocasting is a slurry deposition technique described in U.S. Pat. No. 6,027,326 of J. Cesarano et al. It is capable of producing large-scale, near-net shape components that utilizes feedstock of negligible organic content ( less than 1 vol %). In this approach, pseudoplastic ceramic suspensions (xcfx86solids  greater than 0.50) are deposited onto a substrate in a precise pattern. Upon minimal drying, the as-deposited suspension undergoes a liquid-to-solid transition that freezes in the structure of the patterned element. Current challenges to this approach involve controlling the macroscopic shape evolution of the as-deposited components. Both slumping, which results from insufficient drying in high aspect ratio multi-layer components, and considerable stair-casing in component walls have been observed. Hence, there is a need to develop new feedstock materials with low organic content, such as gelcasting suspensions, to improve the mechanical strength of the deposited layers and thereby overcome these limitations.
Gelation denotes the transition from a liquid (sol) to a solid (gel) state that occurs in the absence of fluid removal. During this process, discrete species in solution undergo growth (e.g., monomersxe2x86x92network polymers; colloidal particlesxe2x86x92aggregated network; or linear polymersxe2x86x92network polymers) as gelation proceeds. Such species grow until at least one cluster, known as a percolating (or spanning) cluster, reaches a critical size on the order of the sample dimensions, signifying the formation of a gel. At sol-gel transition, dramatic changes in the viscoelastic properties of the system are observed, as described by H. Winter, xe2x80x9cPolymeric Gels, Materials that Combine Liquid and Solid Properties,xe2x80x9d MRS Bull., 16 [8] 44-48 (1991). The viscosity of the system increases with time before its divergence to infinity at the gel point, which coincides with the formation of a 3-D space-filling cluster whose characteristic size is on the order of the sample dimensions. If growth is arrested because of the depletion of reactant before gel formation, the system remains in the liquid state, and its apparent viscosity plateaus to a steady-state value. Beyond the gel point, additional linkages form between growing clusters, thereby strengthening the gel network.
A. Fanelli et al., U.S. Pat. No. 4,734,237, Process For Injection Molding Ceramic Composition Employing An Agaroid Gell-Forming Material To Add Green Strength To A Preform, discloses the steps of forming a mixture comprising metal and/or ceramic powders, a gel-forming material having a gel strength, measured at a temperature between 0xc2x0 C. and about 22xc2x0 C. and a gel consisting essentially of about 4 wt. % of the gel-forming material and water, of at least about 100 g/cm2, and a gel-forming material solvent, and molding the mixture at a temperature sufficient to produce a self-supporting article comprising the powder and a gel. Unlike conventional injection molding, this process uses relatively low (xcx9c10 vol % or less) binder content.
Gelcasting is a bulk fabrication technique for producing, near-net shape ceramic components. M. Tanney, U.S. Pat. No. 4,894,194, Method For Molding Ceramic Powders, discloses forming a slurry mixture including ceramic powder, a dispersant for the metal-containing powder, and a monomer solution. The monomer solution includes at least one multifunctional monomer, a free-radical initiator, and an organic solvent. The slurry mixture is transferred to a mold, and the mold containing the slurry mixture is heated to polymerize and crosslink the monomer and form a firm polymer-solvent gel matrix. The solid product may be removed from the mold and heated to first remove the solvent and subsequently remove the polymer, whereafter the product may be sintered.
M. Tanny, U.S. Pat. No. 6,066,279, Gelcasting Methods, provides a solution of HMAM and water. At least one inorganic powder is added to the mixture and an initator system is provided to polymerize the HMAM to form a hydrogel. This system also forms the gel in situ.
It is readily apparent that in situ gelation is not suitable for SFF, as such techniques require a form to maintain the shape of the article until it hardens.
S. Morissette and J. A. Lewis investigated aqueous, alumina-poly(vinyl alcohol) gelcasting suspensions. They found that the addition of particles shifted the sol-gel phase transition to increasingly lower critical cross-linking concentrations, suggesting that solid filler was not inert. They investigated the chemorheological properties of these suspensions and found that the gelation behavior can be tailored by varying the suspension composition (i.e., solids volume fraction, PVA content, and cross-linking agent concentration), as well as the processing temperature. They concluded that polymer-based gelcasting systems are a viable alternative to in situ polymerization of monomeric systems. Also, the strong temperature dependence of the gelation kinetics allows one to handle such systems at low temperature (where gelling is slowed) prior to casting. In addition, they determined that the gel strength can be tailored by exploiting the dependence on cross-link concentration as well as the solids volume fraction dependence, as indicated in their paper, Chemorheology of Aqueous-Based Alumina-Poly(vinyl) alcohol) Gelcasting Suspensions, J. Am. Ceram. Soc. 82 (3) 521-28 (March 1999), which paper is incorporated herein by reference.
It is an object of this invention to provide solid freeform fabrication using a computer-controlled extrusion apparatus where as-cast SFF-derived components exhibit uniform particle packing comparable to bulk gelcast components with minimal macro-defects (e.g., slumping or stair casing) and no detectable micro-defects (e.g., bubbles or cracking).
It is also an object of this invention to use gel-casting suspensions as feedstock material in a SSF system to tailor deposition behavior, and hence, component properties of SFF-derived, advanced ceramic components.
These and other objects of the invention may be attained by a method for freeform fabrication of objects comprising providing a first fluid in a first container, providing a second fluid in a second container; feeding the first and second fluids to a mixing chamber; mixing the first and second fluids in the mixing chamber to form a third fluid; and forming an object by depositing a bead of the third fluid from the mixing chamber onto a platform that moves relative to the mixing chamber, the third fluid gelling shortly after being deposited.